Improving a garden classic - Oregon Association of Nurseries

Growing Knowledge
Improving a garden classic
Targeted breeding efforts with rose-of-Sharon
aim for new combinations of form and color
By Ryan Contreras and
Jason D. Lattier
Rose-of-Sharon, or althea (Hibiscus
syriacus L.), has graced American gardens since colonial times with its large
tropical blooms and bright colors.
Modern cultivars of althea can be
found in gardens from coast to coast
and are prized for their winter hardiness, plethora of colors, and unique
forms including single-flowered, double-flowered, and anemone types.
This fantastic diversity of flowering
traits, combined with the short generation time from seed to flower, provides
an excellent opportunity for breeding.
Noted horticulturist Dr. Michael Dirr
stated, “Hibiscus syriacus is a gold mine
for a breeder because of the potential for
myriad flower forms quickly and fully
expressing their attributes” (Dirr, 2009).
Even with the wealth of cultivars
available, possibilities for new combinations and improvements still exist. We are
not the first to take note of potential in this
species. There have been many before us
who have made substantial improvements
that we hope to build upon.
U.S. National Arboretum cultivars
Unfortunately, many cultivars are
prolific seed producers that self-sow
and can be a nuisance in the garden.
Don Egolf of the U.S. National
Arboretum (USNA) attempted to address
this issue by releasing four cultivars
(‘Aphrodite’, ‘Diana’, ‘Helene’ and
‘Minerva’) that were reported to be
sterile or nearly sterile triploids (Egolf,
1981; 1986; 1988). However, since
their release, these cultivars have been
observed to produce seed.
First in the series to be released in
1970, ‘Diana’ has pure white flowers that
lack eye spots. Egolf’s other three cultivars have prominent eye spots with white
(‘Helene’), lavender-pink (‘Minerva’) and
pink (‘Aphrodite’) flowers.
Cultivar release information for
these cultivars states that they are sterile
or nearly sterile triploids that resulted
from doubling the chromosomes of the
diploid cultivar ‘William R. Smith’ to
develop a tetraploid, which was then
crossed with various diploid cultivars
(Egolf, 1981; 1986; 1988). However, the
natural ploidy level (number of chromosome sets) in rose-of-Sharon is tetraploid (2n = 4x = 80).
If ‘William R. Smith’ was actually
doubled, the result would have been an
octoploid (2n = 8x = 160). Crosses with
untreated tetraploids would have resulted
in hexaploid progeny (2n = 6x = 120).
It is unclear how the original polyploids were identified other than based
on morphology, which is an unreliable
method. Therefore, the ploidy level of
the original treated ‘William R. Smith’
plant is in doubt.
What is particularly interesting
about this story is Egolf was an accomplished cytogeneticist and certainly
understood ploidy manipulation and
its use in a breeding program. In fact,
Egolf’s Ph.D. dissertation at Cornell
A flower of the hexaploid cultivar H. l. ‘Flogi’ (Pink
Giant®), which is being used to develop potentially
sterile cultivars due to their odd ploidy levels.
Photo by Jason Lattier
University (1956) was titled “Cytological
and interspecific hybridization studies in
the genus Viburnum.”
Polyploids
Historically, polyploids were
identified based on their morphology,
which often includes thicker and
darker leaves and flowers that are
often twisted or otherwise malformed.
However, this is not always the case
and relying on gross morphology alone
can lead to misidentification.
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An ongoing series provided by
Oregon State University
in partnership with OAN
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rose-of-sharon
Another tool used by plant breeders to identify polyploids is to measure
specialized epidermal cells known as
stomata. Measuring stomata is useful for
identifying the ploidy level of the epidermis, but provides no information on
the germ layer from which pollen and
eggs are derived. Using either morphology or stomatal measurements may lead
to erroneously identifying plants that
breed as polyploids.
Today, breeders have the advantage
of using flow cytometry to accurately
and quickly identify ploidy levels and
relative genome sizes, a tool that was
not available to Egolf.
Flow cytometry measures fluorescence of plant nuclei to calculate ploidy
and relative genome sizes, and can also
be used to identify chimeras. Without
this tool, it would have been more difficult for Egolf to identify chimeras in
which some of the cells or cell layers
have doubled chromosomes, while the
germ layer remained unaffected.
Still, what remains a mystery is why
the original selections showed such
a marked reduction in fertility if they
were not triploids, or at least plants of
an odd ploidy level.
Assessing fertility and ploidy level
To begin answering that question,
we set out to quantify the levels of male
and female fertility in available cultivars,
including the purported triploids from
the USNA. We performed reciprocal
crosses among nine rose-of-Sharon cultivars being grown in a glasshouse.
Our data showed that there is no
substantial reduction in overall fertility in any of the four USNA cultivars
compared to other industry standards
(Contreras et al., 2013). However, we
did observe very low fertility in the
cultivar ‘Flogi’ (Pink Giant®), which
produced only 0.08 and 0.5 seedlings
when used as a female and male parent, respectively.
We followed up these results with
flow cytometry analyses of these nine
cultivars to determine their ploidy level.
We found that all cultivars, including the
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purported triploids, were
actually tetraploids, with
the notable exception of
Pink Giant, which was a
hexaploid (2n = 6x = 120)
(Contreras et al., 2013).
The fact that this
cultivar with higher
ploidy level exhibits
reduced fertility supports the idea that ploidy
manipulation in rose-ofSharon remains a viable
option for developing sterile cultivars. After its identification as a hexaploid,
Pink Giant has become
an important parent in
our breeding program.
Research greenhouse at Oregon State University, where numerous
commercial cultivars of rose-of-Sharon are being used in controlled
crosses. Photo by Jason Lattier
Exciting new phenotypes
Of course, sterile cultivars will be
of great utility to gardeners, but seedless cultivars do not necessarily get
one’s heart racing!
To create excitement, we are working toward new combinations of flower
colors and forms. With a flurry of new
releases on the market, it may be said
that there has never been a better time
to be a rose-of-Sharon breeder.
With little effort we put together a
germplasm of nearly 30 top-performing
cultivars representing many combinations of colors, flower forms, and presence or absence of eye spots. This
collection includes cultivars that have
been commercially available for some
time as well as many new releases
including the garden dwarf Lil’ Kim™
from Monrovia, four Smoothie™ altheas
from Garden Debut, and four cultivars
in the island series (Hawaii™, Bali™,
Fiji™ and Tahiti™) from First Editions.
As our breeding program grows,
we will continue to incorporate the full
array of new colors and forms from current nursery releases.
But, how does one go about developing a specific new look? Breeding
new nursery crops often involves knowledgeable experts (aka plant geeks)
identifying new phenotypes that arise by
chance mutations in large nursery populations. In addition, new phenotypes
often are identified by serendipitous
genetic recombination occurring through
open pollination on the nursery or farm.
There is nothing wrong with this
process. Thousands of introduced cultivars are evidence of how effective
it can be. But is there a better way to
improve rose-of-Sharon? Can we set
out to achieve a dwarf cultivar that has
pure white double flowers with no eye
spots? We think the answer is yes.
We are working on assessing the
inheritance of traits including plant
height, flower color, flower texture (single, anemone, double), and eye spot.
Crosses during 2012 and 2013
consisted of nearly 100 unique combinations of both older and modern
cultivars. Flower traits such as single,
anemone and double flowers in all
color combinations were included in
these crosses. Other unique traits, such
as flowers lacking eye spots, plants with
dwarf habits, and plants with different
ploidy levels, also were crossed in different combinations.
Pollinations were made in a controlled greenhouse to exclude pollinators. Accidental self-pollinations were
easily avoided because flowers of roseof-Sharon have heavy, sticky pollen that
is physically separated from the recep-
tive stigma. Seeds in 2012 and 2013
were collected, counted and germinated
over winter. Nearly 1,200 seedlings
were field planted and are currently
being evaluated for form, flower color
and reduced fertility.
Evaluating progeny from hybrid
crosses allows breeders to make predictions about progeny obtained from
future crosses between specific parents.
Self-pollinations and reciprocal crosses
were made for most parents in our
crossing study.
Reciprocal crosses, or crosses
between two plants using each plant
as both male and female parent, allow
breeders to determine maternal or
paternal effects on gene expression.
Self-pollinations are useful because
they allow you to elucidate the variability of a trait in the plant’s genome.
For instance, if you self-pollinate a
white-flowered rose-of-Sharon and all
of the progeny produce white flowers,
then you can predict that your parent
is true breeding for white flowers. If
you cross the same plant with a blueflowered rose-of-Sharon and find that
only a small fraction of the progeny
have white flowers, you can predict
that white flowers are weakly expressed
(recessive) when combined with the
genes from the blue-flowered parent.
No formal studies have been published on inheritance patterns in roseof-Sharon; however, Dirr (2009) reports
that an open-pollinated ‘Blue Bird’ produced progeny that flowered in a range
of colors (white, pink, rose, lavender
and blue). It is unclear if this pattern
of inheritance is due to segregation of
genes present in ‘Blue Bird’ or if perhaps the pattern Dirr observed was due
to outcrossing.
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Meeting the demand for new cultivars
These types of progeny tests have
been used since the days of Mendel
and will allow us to determine the
inheritance patterns for flower colors,
forms and eye spots in rose-of-Sharon.
Knowing how traits are inherited will
allow us to utilize a targeted breeding
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rose-of-sharon
approach to meet the demands for new
cultivars in the nursery industry.
Long-term evaluations of hybrids
will allow us to calculate combining abilities for parents and identify
hybrids with improved growth habits
and increased bloom time. In the years
ahead, we hope to provide gardeners
new combinations of color, form and
texture on sterile, hardy shrubs that
behave well in the home garden.
Ryan Contreras, Ph.D., is an assistant
professor of ornamental plant breeding in the Oregon State University
Department of Horticulture. He can
be reached at 541-737-5462 or
[email protected].
Jason D. Lattier is a graduate research
assistant in the Oregon State University
Department of Horticulture. He can be
reached at [email protected].
References
Contreras, Ryan, Mara Friddle and
Jason D. Lattier. (2013). Relative fertility and ploidy levels of selected rose
of Sharon cultivars. Southern Nursery
Association Research Conference
Proceedings, 58: 232–236.
Dirr, Michael A. (2009). Manual
of woody landscape plants: Their identification, ornamental characteristics,
culture, propagation, and uses (6th ed.).
Champaign, IL: Stipes Publishing.
Egolf, D.R. (1970). Hibiscus syriacus ‘Diana’, a new cultivar [Malvaceae].
Baileya, 17: 75–78.
Egolf, Donald R. (1981). ‘Helene’
rose of Sharon. HortScience, 16: 226–227.
Egolf, Donald R. (1986). ‘Minerva’
rose of Sharon. HortScience, 21:
1463–1464.
Egolf, Donald R. (1988). ‘Aphrodite’
rose of Sharon. HortScience, 23: 223.
A small sample of the amazing diversity of flower
colors and forms found in rose-of-Sharon.
Photo by Jason Lattier
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